1. Field of the Invention
The present invention relates to the filed of the digital signal processing, and more specifically, to the field of QAM modems.
2. Discussion of the Prior Art
When the communication exists for a long period of time, it is possible to use a conventional phase-lock loop (PLL) in a receiver to recover the carrier from a continuously transmitted signal. The PLL that is specifically designed to recover the carrier is termed as a phase carrier loop.
However, in a time-division multiple access (TDMA) communication system, or in any other communication system that requires a rapid carrier acquisition, the available spectrum is used by letting each user to have access to the whole band for a short time (traffic burst), during which time the user transmits data as fast as the user can. The user's frequency allocation is shared with the other users who have time slots allocated at other times.
Thus, in a TDMA-type burst communication systems, or in any other communication system that requires a rapid carrier acquisition, the signals exist for a short period of time. Consequently, there is a need to quickly determine the carrier frequency offset during a net entry time period. The recovered frequency offset allows one to increase the speed of locking the tracking loop onto the incoming burst carrier signal.
What is needed is to design a QAM carrier tracking loop system that is capable of quickly determining the carrier frequency offset during the net entry time period, and to utilize the recovered frequency offset in order to increase the speed of locking the carrier tracking loop onto the incoming burst carrier signal.